On the SitePoint Web Tech blog Raena Jackson-Armitage has a recent post on a few tools that you can use to mimic some of the functionality Twitter offers (three of them) with applications that already exist.

If you're anything like me, you've already wondered how easy it would be to make your own'"perhaps you'd like to set up a microblog for you and your colleagues to share links and have discussions. [...] Today we'll look at three new solutions that are easy to set up, have reasonably modest hosting requirements, and even include some features that blow Twitter out of the water.

Identi.ca is a microblogging service. Users post short (140 character) notices which are broadcast to their friends and fans using the Web, RSS, or instant messages. It runs on the Free Software Laconica tool.